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Jupiter's four major moons, discovered by Galileo 401 years ago now, are fascinating studies in contrast. Heat from tidal forces have shaped the inner two moons, giving Io active volcanoes and probably providing Europa with ice-covered oceans. The furthest of the four, Callisto, appears to have frozen as it formed. The remaining moon, Ganymede, was a bit of a mystery. Like Callisto, it appears to be a mixture of rock and water, with a big difference: on Ganymede, the water appears to have melted, allowing the rock to sink to the core, and then froze up again. Two scientists from the Southwest Research Institute now think they know why: Ganymede was hit so hard by asteroids and comets during the solar system's early history that it underwent runaway melting.

Given the fact that Callisto and Ganymede have such similar densities (they differ by about 0.1 grams per cubic centimeter), scientists have long assumed that they share similar compositions, primarily a mixture of rock and ice. Callisto's surface is much like our Moon's, dominated by impact craters, and with little sign of geologic activity. Ganymede's couldn't be more different, with obvious rocky and icy areas, and many surface grooves that indicate the moon has been extensively remodeled.

The tidal forces that shape the inner moons are driven by the constantly shifting gravitational pulls of Jupiter and the outer moons. But Ganymede is far enough out that these forces are significantly weaker. Although it's possible to model a composition where it would partly melt and Callisto wouldn't, the conditions where this would occur are very precise, and these models don't do a good job of explaining why it seems to have melted only once in its history.

The authors of the new paper focus on a period early in our Solar System's history called the Late Heavy Bombardment. The record of impacts on the Moon suggest that gravitational disturbances sent about 1.6 x 1022g of material into its surface, equally divided between comets and asteroids. That implies the disruption of a planetesimal disk that contained about 20 times the Earth's mass, most likely via gravitational interactions with the gas giants.

Obviously, a gravity well like Jupiter's would attract a significant number of these bodies. More significantly, the planet would focus their trajectories such that the closer the moon to Jupiter, the more impacts it would receive. According to the authors, the Late Heavy Bombardment would strike Ganymede with about 80 times the mass than Callisto was hit with. They estimate that the total energy imparted to Ganymede could be up to 1036erg, enough to melt its ice five times over and allow all of the rocky material to sink to the core.

Obviously, however, all of this energy did not arrive at once, and not all of it was converted to melting its ice. So the authors built a model of impact-driven melting and core formation. In the model, impacts create a spherical melted area below the surface, allowing the rock in that region to plunge through to the bottom of the melt. The impact of these rocks on the bottom releases further heat, which can set off what they call "runaway differentiation"—the complete melting of the moon's ice and formation of a rocky core.

The neat thing about this model is that, to have this process melt Ganymede, there had to be a minimal amount of mass involved in the Late Heavy Bombardment. For it not to have done the same to Callisto, the Bombardment couldn't have been too large. Combined (and given a few assumptions about the density of the asteroids and comets that were involved), and the two moons set upper and lower limits on the material that created the Late Heavy Bombardment.

That number turns out to be somewhere between six and 23 times the mass of Earth. That nicely encompasses the mass estimated by examination of the craters on the Moon, which is precisely what we like to see when we're doing science: two pieces of evidence, derived from different data and using different assumptions, that point in the same direction.

The fact that we also like to think about big things going boom is just an added bonus.